Coach Bo Pelini announced Thursday night that Jackson would require season-ending surgery to repair a knee injury. After a breakout spring, Jackson started camp well Monday with several head-turning plays from the nickel position.

His progress ended abruptly.

One of Nebraska's top athletes, Jackson factored heavily on special teams in 2012 and 2013 but failed to earn significant time in the secondary as he struggled with defensive concepts. He turned a corner in March and April.

The nickel spot in Pelini's scheme has long served as a key spot to earn mismatches and create big plays. Ciante Evans performed well in the spot last year.

Jackson, because of his athleticism, promised to add an important spark to a secondary faced with the loss of cornerback Stanley Jean-Baptiste and safety Andrew Green in addition to Evans.

The Huskers must now incorporate another newcomer. Junior-college transfer Byerson Cockrell, who played nickel and cornerback in the spring after joining the Huskers in January, is the favorite to fill Jackson's role.

"I love Byerson Cockrell," Pelini said Thursday. "He is a really good player. He is a very smart and very intelligent player."

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True freshman Joshua Kalu will also get a look this month at nickel, Pelini said.

Kalu starred at Houston's Alief Taylor High School, a Texas 5A power. Regardless, the thought of a starting nickel with no experience at the FBS level may lead to a restless month for first-year secondary coach Charlton Warren.

For Jackson, the excruciating wait continues. He hasn't played a full game since his senior year of high school at Spring (Texas) Klein Collins in 2010.

This was supposed to the year. It came to a cruel end in the first week of practice. And the most inexperienced area of the Nebraska defense just grew a little more green.

ESPN Sport Science - Randy Gregory Ruins A Tackling Dummy

NFL Draft prospect Randy Gregory delivered a hit that knocked the head of a tackling dummy, one of the hardest hits ever measured in the lab. To see more of how Gregory fared, check out the ESPN Sport Science draft combine special on April 23.

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ESPN Sport Science - Randy Gregory Ruins A Tackling Dummy

NFL Draft prospect Randy Gregory delivered a hit that knocked the head of a tackling dummy, one of the hardest hits ever measured in the lab. To see more of how Gregory fared, check out the ESPN Sport Science draft combine special on April 23.